# not equal to $$e^{ x }$$

Calculus Level pending

$$Let\quad f(x)=\quad 1+\frac { x }{ 1! } +\frac { { x }^{ 2 } }{ 2! } +\frac { { x }^{ 3 } }{ 3! } +\frac { { x }^{ 4 } }{ 4! }$$ Then the number of real roots of f(x) is/are

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